Direct surface patterning of carbon

ABSTRACT

The present invention provides a method for producing carbon structures by laser irradiation, the method comprising: (i) providing a substrate, at least a portion of whose surface being covered with a sample comprising one or more thermally degradable organic compounds, said sample being in the form of homogeneous solution, suspension or emulsion; (ii) irradiating said covered surface portion locally by applying a focused laser beam, thus resulting in local deposition of carbon, and (iii) repeating step (ii) by moving either the laser beam or the sample, thus creating a desired pattern of carbon structures. The present invention further provides carbon structures produced by the method of the invention.

FIELD OF THE INVENTION

[0001] The present invention relates to a method for producing patternsof carbon.

[0002] List Of References

[0003] In the following description reference will be made to severalprior art documents shown in the list of references below. The referencewill be made by indicating in brackets the first author name from thelist.

[0004] A. E. Aleksenskii, M. V. Baidakova, A. Ya. Vul, V. Yu. Davydov,Yu. A. Pevtsova, Phys. Solid State 1997, 39, 1007.

[0005] J. C. Angus, H. A. Will, W. S. Stanko, J. Appl. Phys. 1968, 39,2915.

[0006] F. P. Bundy, H. T. Hall, H. M. Strong, R. H. Wentorf Jr., Nature1955, 176, 51.

[0007] M. Elbaum, D. Zbaida, E. Klein, A. Lachish-Zalait, WO 01/38940A2.

[0008] Y. Gogotsi, S. Welz, D. A. Ersoy, M. J. McNallan, Nature 2001,411, 283.

[0009] A. Lachish-Zalait, D. Zbaida, E. Klein, M. Elbaum, Adv. Funct.Mater. 2001, 11, 218.

[0010] Y. Namba, E. Heidarpour, M. Nakayama, J. Appl. Phys. 1992, 72,1748.

[0011] L. C. Qin, D. Zhou, A. R. Krauss, D. M. Gruen, NanoStructuredMaterials 1998, 10, 649.

[0012] J. Robretson, Prog. Solid State Chem. 1991, 21, 199.

[0013] K. Uetake, N. Sakikawa Chem Abstr., 1974, 81:5175t

[0014] X. Wang, J. Chen, Z. Zheng, Z. Sun, F. Yan, J. Crystal Growth1997, 181, 308.

[0015] M. Yoshikawa, Y. Mori, H. Obata, M. Maegawa, G. Katagiri, H.Ishida, A. Ishitani, Appl. Phys. Lett. 1995, 67, 694.

BACKGROUND OF THE INVENTION

[0016] Carbon is a very versatile element, due to the different ways inwhich carbon atoms can bond to each other and to other elements. Themost common naturally occurring forms of pure carbon are graphite anddiamond.

[0017] In graphite, the atoms are threefold coordinated as sp² hybrids,forming planes of six-member rings. Carbon atoms bond strongly to eachother within a plane but weakly between adjacent planes. Graphite isopaque, soft, flexible, and an excellent conductor of heat andelectricity. These properties are exploited in foundries, lubricants,brake linings, crucibles and pencils.

[0018] The diamond allotrope consists of four fold-coordinated carbonatoms (sp³ hybrids) and the atomic bonding is strong in all directions.Diamond is the hardest known material, electrically insulating, andtransparent from the far ultra-violet to the far infrared. Beside beinga precious stone, potential applications include wear-resistantcoatings, thin films semiconductor devices, heat sinks, abrasives andcutting tools.

[0019] The currently preferred method for preparing artificial diamondcrystals is based on high-pressure high-temperature (HPHT) (Bundy etal., 1955), which simulates conditions like those of natural creation ofdiamond. Although pure diamond is obtained, bulk crystallites areproduced, and due to its hardness it is difficult to model them intoshapes required in many applications. In order to facilitate the use ofthe diamond properties, the chemical vapor deposition (CVD) method(Angus et al., 1968) was developed, according to which the carbon atomscondense, out of the vapor phase, in diamond configuration on a heatedsubstrate at low pressure.

[0020] Another form of carbon known as diamond-like carbon (DLC) films(Robretson, 1991) consists of amorphous carbon that contains asignificant fraction of sp³ bonding. DLC films or films prepared by CVDcontain mixtures of sp² and sp³ carbon forms in different proportions.These films have important technological applications due to theirhardness and chemical inertness, low electronic affinity and wideoptical gap. The films can be exploited for optoelectronic deviceapplications, panel display, protective coating, wear resistant coating,abrasive for semiconductors, reinforce for polymer and rubber, and heatsinks.

[0021] The exploitation of carbon forms for microelectronics and sensorapplications as well as protective coating of electrical circuits, isstill limited by the lack of a suitable means of carbon lithography ordirect carbon patterning.

[0022] It has been published earlier by co-inventors (Lachish-Zalait etal., 2001 and Elbaum et al in WO 01/38940) that microscale-patternedsurfaces can be generated by applying a tightly focused single-modelaser beam tightly focused through an optical lens directly on ahomogeneous solution containing soluble metal salts or metal compounds.The laser beam strikes the dissolved chemicals in a confined volume anda localized microchemistry process takes place (oxidation-reduction,chemical or thermal decomposition) at the glass/solution interface.Consequently, the product deposits as a solid metal or metal compoundand firmly attaches to the substrate that holds the solution. Operatingthe laser while moving the microscope stage or the laser beam drawcontinuous micro-scale lines. This direct micropatterning was applied ona variety of precursor solutions leading to patterns of metallic silver,gold, platinum, oxidized copper as well as compounds containingtransition metals-II, for example Mo and W.

SUMMARY OF THE INVENTION

[0023] It has been found, according to the present invention, thatcarbon microstructures may be produced by applying a focused laser beamdirectly onto a sample in the form of a homogeneous solution, dispersionor emulsion, of heat degradable precursors. The carbon structures soformed, show evidence of both sp³ and sp² hybridization and their sizeis in the micron range or even smaller. The laser beam induces thermaldecomposition of the heat degradable precursor in solution, thusaffording the precipitation of carbon patterns that attach firmly to thesurface that holds the irradiated solution.

[0024] Thus, the present invention relates to a method for producingcarbon structures by laser irradiation, the method comprising:

[0025] (i) providing a substrate, at least a portion of whose surface iscovered with a sample comprising one or more thermally degradableorganic compounds, said sample being in the form of homogeneoussolution, suspension or emulsion;

[0026] (ii) irradiating said covered surface portion locally by applyinga focused laser beam, thus resulting in local deposition of carbon, and

[0027] (iii) repeating step (ii) by moving either the laser beam or thesample, thus creating a desired pattern of carbon structures.

[0028] The deposition of carbon is obtained from the decomposition ofthe thermally degradable compounds comprised by the irradiated sample.

[0029] The carbon patterns formed by the method of the present inventionmay contain various amounts of sp³ and sp² bonding, thus having physicalcharacteristics ranging from diamond, diamond-like carbon and graphite.

[0030] In the above method, the scanning of the surface with the lasermay also be carried out in a predetermined manner as a direct-writetechnique. Direct-write patterning is ideal for sample-specific marking,such as serial numbers, codes, identification cards, etc.

[0031] Examples of thermally degradable organic compounds are peroxides,azo compounds, acids, ketones, diketones, biphenyls and polyphenylcompounds.

[0032] The wavelength of the laser beam may be in the visible, UV, IR ornear IR range. The patterning with IR lasers was accomplished withsuccess, as described below and this result is surprising since from amechanistic point of view the infrared laser photons do not havesufficient energy to break chemical bonds.

[0033] The major factors influencing the patterning rate are theprecursor, the solvent, the laser power and the type of the substrate.The size and shape of the structure generated on the surface is dictatedby the width of the laser spot and the thermal diffusion rate. Arrays ofany desired shape may be built by serial production of a local pattern(e.g. dot or line).

[0034] The carbon patterns produced by the method of the presentinvention may be utilized in a number of novel applications. Patterns ofcarbon with sp³ hybridization can serve as nucleation sites for growingdiamond on surfaces not amenable to direct deposition. In addition,carbon patterns can be drawn on a substrate and cast into a polymermatrix. Peeling off the matrix from the glass substrate will afford 3Dmicron size channels suitable for micro-fluidic applications. Also, theability to pattern carbon with sp³ hybridization can be used in electronemitters of high quantum efficiency. The ability to pattern then opensapplications in device development for detectors.

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to understand the invention and to see how it may becarried out in practice, a preferred embodiment will now be described,by way of non-limiting example only, with reference to the accompanyingdrawings, in which:

[0036]FIG. 1 shows an optical image with low magnification of the carbonpattern.

[0037]FIG. 2 shows imaging in a field emission scanning electronmicroscope (SEM; FEI model XC-30) using the environmental mode with 1torr water vapor pressure without any coating.

[0038]FIG. 3 shows a typical electron diffraction of the carbon linewith sp³ configuration and spotty morphology.

[0039]FIG. 4 shows a typical selected site in the carbon line of anordered array of graphene sheets with lattice fringes (marked betweenarrows) of 3.35 Å, corresponding to carbon with sp² configuration.

DETAILED DESCRIPTION OF THE INVENTION

[0040] It has been found according to the present invention that carbonwith various amounts of sp³ and sp² hybridization may be obtained, byapplying a tightly focused laser beam directly onto a sample comprisingheat degradable precursors, the sample being in the form of homogeneoussolution, suspension, or emulsion. The sample may be held by varioussurfaces, both hydrophilic and hydrophobic in nature, and the surfacesdo not require any pretreatment such as cleaning, degreasing, and thelike, before the application of the sample.

[0041] In practice, a glass slide and cover slip enclose the sample. Thefocused laser radiation passes through the glass cover slip and strikesthe confined sample at the glass/solution interface. Deposition mayoccur on either glass surface on which the laser is focused. Thermaldecomposition of the heat degradable precursor in solution, suspensionor emulsion form is induced, thus affording the precipitation of carbonthat contains various amounts of sp³ and sp² hybridization.

[0042] The lasers used in the method of the present invention mayoperate in the visible, UV or infra red (1R) region, more preferably inthe IR region (830, 980 nm) and are ideally focused to a narrow spot bya microscope objective. The intensity of the laser beam at the sampleinterface was measured and found to be on the order of 10 mW.

[0043] Patterning of the substrate takes place only if the laser isfocused directly at the substrate-solution interface. Defocused light orfocusing within the bulk solution has no apparent effect. Both the nearand the far surface with respect to the objective can be patterned.

[0044] In the initiation step, it was observed that directing the laserbeam to a very small amount (about 0.5-1 micron) of freshly precipitatedcrystals of dibenzoyl peroxide could induce initiation of carbondeposition, although the starting material has no significant absorptionat the laser wavelength of irradiation. The same applies to all startingmaterials used in the method of the invention, when the irradiatinglaser operates in the IR or near IR region.

[0045] However, once initiated, it appears that the deposited productabsorbs the laser radiation. The deposition of the carbon involvesintense local heating. Violent bubbling was observed in the fluid, and atrace of molten borosilicate glass substrate was seen in the SEM aftermechanical removal of the deposited carbon line. Local melting was alsoobserved on quartz cover slip, indicating a local temperature exceeding1400° C. Thus, once the process of deposition begins, its propagation isself-sustaining.

[0046] For the case of dibenzoyl peroxide as a precursor, it wasobserved that an alcohol solution of this compound undergoes thermaldecomposition at around 70-80° C. Thermal decomposition of dibenzoylperoxide is known to produce carbon dioxide, benzoic acid, biphenyl,phenyl benzoate, benzene and terphenyls (Uetake et al., 1974). Theproducts, most probably, are obtained through the appropriate radicals,optionally followed by dimerization step. In the present case of carbonpatterning, it is suggested that a complete pyrolysis takes placeleading to the deposition of pure carbon on the glass surface. Operatingthe laser momentarily forms isolated spots, while moving the microscopestage or the laser spot in the x-y plane draws continuous lines. Therate of deposition depends on the identity of the precursor solution,laser power, and deposition speed.

[0047] Examples of thermally degradable compounds are peroxides, azocompounds, acids, ketones, diketones, biphenyls and polyphenylcompounds. More specifically, compounds suitable to be used in themethod of the invention are dibenzoyl peroxide, di-tertbutyl peroxide,azo-bis-isobutyronitril, benzophenone, benzoic acid, dibenzoyl,benzhydrol, ethyl benzoate, benzoyl benzoate, biphenyl, p-terphenyl,naphthalene, anthracene, camphor, etc.

[0048] In practice, a precursor solution is obtained by dissolving oneor more thermally degradable compounds into solvents capable to dissolvethese compounds, such as alcohol (e.g. ethanol or isopropyl alcohol),toluene, benzene, xylene, etc. For example, a dibenzoyl peroxidesolution was prepared by dissolving dibenzoyl peroxide (30% in water)(90 mg) in ethanol (3 ml) or in toluene (1 ml). Alternatively, thesample is prepared in the form of suspension or emulsion, by dissolvingone or more thermally degradable compounds into a suitable solvent orsolvent system.

EXAMPLES

[0049] Experimental

[0050] An IR diode laser source was used in a configuration of opticaltweezers. The laser operated at two wavelengths: at 830 and 980 nm andthe output power at the sample was 5 and 10 mW, respectively.

[0051] Scanning electron microscope was performed after coating thesample with carbon and gold in a JEOL GMC 6400 equipped with an OxfordLink EDS spectrometer. Alternatively, imaging was performed in a fieldemission scanning electron microscope (SEM; FEI model XC-30) usingenvironmental mode with 1 torr water vapor pressure without any coating.

[0052] Electron diffraction of the removed lines adsorbed dry ontocarbon/collodion coated Cu grid, were carried out by transmissionelectron microscope (TEM; Philips model CM 120).

[0053] The sample cell consisted of a long cover slip (22×40 mm), fixedcrosswise to an ordinary glass microscope slide (25×76 mm) using waxspacers, leaving final dimensions approximately 5×25×0.1 mm).

[0054] A precursor solution was prepared by mixing one or more heatdegradable precursor in a suitable solvent or mixture of solvents andthen it was injected into the sample cell.

[0055] Patterns deposited on the glass slide were thoroughly washed withethanol (×1), toluene (×3), incubated in toluene ×2 times for 1 houreach, to remove the precursors and then dried in air.

[0056] Pattern Characterization

[0057] Patterns were generated with a typical width of 5-7 microns and1.5-2 micron height. Typical deposition speed is about 1 micron persecond depending on the laser power, precursor solution, and thesubstrate glass. In all cases, gray colored lines were observed. FIG. 1shows an optical image of a carbon continuous pattern in the form of aline. Moving the microscope stage or the laser beam, continuous depositsare formed to a length of centimeters.

[0058] The elemental analysis as revealed by the electron dispersionmicroscopy (EDS) showed that, in the pattern formed by the method of theinvention, carbon is the dominant element with some traces of oxygen(0.7%). Highly ordered pyrolitic graphite (HOPG) as a standardcalibration for EDS, showed carbon with a content of 0.5-0.7% oxygen.

[0059]FIG. 2 shows a deposited line as imaged in field-emission scanningelectron microscope (SEM; FEI model XC-30), using the environmental modewith 1 torr vapor pressure shows a densely packed line.

[0060] Carbon patterns were deposited on borosilicate glass as well asquartz and mica. In order to establish the electron diffraction pattern,deposited lines were scraped from the glass surface and adsorbed dryonto Cu grid for transmission electron microscopy. Selected areaelectron diffraction (SAED) patterns were recorded from regions wherethe grains were thin enough to remain electron-transparent.Alternatively, the lines were embedded in epoxy resin and sectioned intothin (50-70 nm) slices by an ultramicrotome.

[0061] Two different morphologies have been observed and distinctelectron diffraction (ED) patterns from different sites on the carbonpattern. FIG. 3 shows the most dominant image of the spotty morphologyand two diffraction rings in the ED pattern. The intense ringcorresponds to the interplanar d-spacing of 2.06 Å and a more diffuseline to d=1.22 Å, corresponding to the Miller indices of (111) and (220)of cubic (sp³) diamond, respectively (Namba et al., 1992 and Qin et al.,1998).

[0062]FIG. 4 shows a selected site, one of several, in a patternedsample with an ordered array of graphene sheets with lattice fringes(marked between arrows) of 3.35 _ (Gogotsi et al., 2001). Thediffraction pattern showed an intense ring at spacing of 3.35 _corresponding to the (002) lattice plane of graphite. The other diffuserings are at 2.13 and 1.23 _ corresponding to (100) and (110) latticeplanes of (sp²) graphite, respectively (Qin et al., 1998).

[0063] For comparison, electron diffraction of three samples wasanalyzed as standard controls. Highly ordered pyrolytic graphite (HOPG),showed a typical spotty diffraction pattern corresponding to interplanard-spacing 3.35, 2.07, 1.50, 1.22, 1.15 and 1.10 Å, corresponding to theMiller indices (002), (101), (103), (110), (112) and (006) respectively,typical to sp² graphitic carbon form. In addition, we have recorded thediffraction pattern of synthetic polycrystalline diamond, gray powderwith 1-micron crystallite size. Five spotty rings with d-spacing of2.06, 1.26, 1.07, 0.82, and 0.71 Å. Natural monocrystalline diamondpowder with 1-micron crystallite size showed patterns of singlecrystals. Typical diffraction patterns of several crystals in the samplewith different orientations were with d-spacing at 1.25 Å for onecrystal, 2.07 Å from another, and 1.26 and 1.08 Å from the third one.

[0064] The crystallite size in a patterned sample of the invention wasestimated by fitting the diffraction maximum at 2.06 Å to a Gaussianline shape. The average size calculated from the peak half width usingthe Selyakov-Scherrer expression (Aleksenskii et al., 1997) was found tobe 9-30 Å. Therefore, the derived particle size and crystal qualityinfluence the electron diffraction pattern.

[0065] The Raman analysis is known as a convenient tool to characterizecarbon materials. The Raman spectra (using Renishaw microscope whileexcitation of the sample with HeNe laser at 633 nm or UV at 244 nm) isnot applicable for small (9-30 Å) crystallite (Yoshikawa 1995 and Wang1997) and does not contradict the existence of both sp² and sp³ carbonform.

[0066] In summary, the present invention provides a method fordeposition of carbon patterns on surfaces. The deposited materialcontains high proportion of sp³ and sp²-bonded carbon. The ability todeposit micro-scale patterns opens possibilities for applications tomicromechanical, microelectronic and sensing devices. The method of theinvention may also be used as a direct-write technique.

1-15. (cancelled)
 16. A method for producing carbon structure by laserirradiation, the method comprising: (i) providing a substrate, at leasta portion of whose surface being covered with a sample comprising one ormore thermally degradable organic compounds, said sample being in theform of homogenous solution, suspension or emulsion; (ii) irradiatingsaid covered surface portion locally by applying a focused laser beam,thus resulting in local deposition of carbon, and (iii) repeating step(ii) by moving either the laser beam or the sample, thus creating adesired pattern of carbon structures, wherein said carbon structurescomprise sp³ and sp² bonding.
 17. The method according to claim 16 wherethe deposition of carbon structures is obtained from the decompositionof the thermally degradable compounds comprised by the irradiatedsample.
 18. The method according to claim 16, wherein steps (ii) and(iii) are carried out in a direct-write technique.
 19. The methodaccording to claim 16, where said thermally degradable compounds areselected from the group consisting of peroxides, azo compounds, acids,ketones, diketones, biphenyls, polyphenyl compounds and mixturesthereof.
 20. The method according to claim 19, wherein said thermallydegradable compounds are selected from dibenzoyl peroxide, di-tertbutylperoxide, azo-bis-isobutyronitril, benzophenone, benzoic acid,dibenzoyl, benzhydrol, ethyl benzoate, benzoyl benzoate, biphenyl,p-terphenyl, naphthalene, anthracene and camphor.
 21. The methodaccording to claim 16 wherein the irradiating laser operates in theinfrared or near infrared region.
 22. The method according to claim 21,wherein the irradiating laser is in optical tweezers configuration. 23.Carbon structure comprising sp³ and sp² bonding, produced by laserirradiation of a surface portion of a substrate, said surface portionbeing covered with a sample comprising one or more thermally degradablecompounds that degrade under laser irradiation, where said sample is inthe form of a homogenous solution, suspension or emulsion.
 24. Carbonstructure according to claim 23, wherein said thermally degradablecompounds are selected from the group consisting of peroxides, azocompounds, acids, ketones, diketones, biphenyls, polyphenyl compoundsand mixtures thereof.
 25. Carbon structure according to claim 23 whereinsaid thermally degradable compounds are selected from the groupconsisting of dibenzoyl peroxide, di-tertbutyl peroxide,azo-bis-isobutyronitril, benzophenone, benzoic acid, dibenzoyl,benzhydrol, ethyl benzoate, benzoyl benzoate, biphenyl, p-terphenyl,naphthalene, anthracene and camphor.
 26. Carbon structure produced bythe method of claim
 16. 27. Article of manufacture comprising carbonstructure produced by the method of claim
 16. 28. Article of manufacturecomprising carbon structures according to claim 23.